90 lines
3.1 KiB
Groff
90 lines
3.1 KiB
Groff
.TH pgm 5 "12 November 1991"
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.SH NAME
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pgm - portable graymap file format
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.SH DESCRIPTION
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The portable graymap format is a lowest common denominator grayscale
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file format.
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.IX "PGM file format"
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The definition is as follows:
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.IP - 2
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A "magic number" for identifying the file type.
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A pgm file's magic number is the two characters "P2".
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.IX "magic numbers"
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.IP - 2
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Whitespace (blanks, TABs, CRs, LFs).
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.IP - 2
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A width, formatted as ASCII characters in decimal.
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.IP - 2
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Whitespace.
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.IP - 2
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A height, again in ASCII decimal.
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.IP - 2
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Whitespace.
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.IP - 2
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The maximum gray value, again in ASCII decimal.
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.IP - 2
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Whitespace.
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.IP - 2
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Width * height gray values, each in ASCII decimal, between 0 and the specified
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maximum value, separated by whitespace, starting at the top-left
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corner of the graymap, proceeding in normal English reading order.
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A value of 0 means black, and the maximum value means white.
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.IP - 2
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Characters from a "#" to the next end-of-line are ignored (comments).
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.IP - 2
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No line should be longer than 70 characters.
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.PP
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Here is an example of a small graymap in this format:
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.nf
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P2
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# feep.pgm
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24 7
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15
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0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
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0 3 3 3 3 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 15 15 15 0
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0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 15 0
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0 3 3 3 0 0 0 7 7 7 0 0 0 11 11 11 0 0 0 15 15 15 15 0
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0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 0 0
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0 3 0 0 0 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 0 0 0 0
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0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
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.fi
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.PP
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Programs that read this format should be as lenient as possible,
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accepting anything that looks remotely like a graymap.
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.PP
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There is also a variant on the format, available
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by setting the RAWBITS option at compile time. This variant is
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different in the following ways:
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.IX RAWBITS
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.IP - 2
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The "magic number" is "P5" instead of "P2".
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.IP - 2
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The gray values are stored as plain bytes, instead of ASCII decimal.
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.IP - 2
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No whitespace is allowed in the grays section, and only a single character
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of whitespace (typically a newline) is allowed after the maxval.
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.IP - 2
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The files are smaller and many times faster to read and write.
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.PP
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Note that this raw format can only be used for maxvals less than
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or equal to 255.
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If you use the
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.I pgm
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library and try to write a file with a larger maxval,
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it will automatically fall back on the slower but more general plain
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format.
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.SH "SEE ALSO"
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fitstopgm(1), fstopgm(1), hipstopgm(1), lispmtopgm(1), psidtopgm(1),
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rawtopgm(1),
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pgmbentley(1), pgmcrater(1), pgmedge(1), pgmenhance(1), pgmhist(1), pgmnorm(1),
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pgmoil(1), pgmramp(1), pgmtexture(1),
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pgmtofits(1), pgmtofs(1), pgmtolispm(1), pgmtopbm(1),
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pnm(5), pbm(5), ppm(5)
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.SH AUTHOR
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Copyright (C) 1989, 1991 by Jef Poskanzer.
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.\" Permission to use, copy, modify, and distribute this software and its
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.\" documentation for any purpose and without fee is hereby granted, provided
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.\" that the above copyright notice appear in all copies and that both that
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.\" copyright notice and this permission notice appear in supporting
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.\" documentation. This software is provided "as is" without express or
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.\" implied warranty.
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